This invention relates to electrostatographic reproducing apparatus and more particularly to a novel corona generating device, together with method for using such a device to improve copy quality.
In an electrostatographic reproducing apparatus commonly used today, a photoconductive insulating member is typically charged to a positive potential, thereafter exposed to a light image of an original document to be reproduced. The exposure discharges the photoconductive insulating surface in exposed of background areas and creates an electrostatic latent image on the member which corresponds to the image areas contained within the original document. Subsequently, the electrostatic latent image on the photoconductive insulating surface is made visible by developing the image with a developing powder referred to in the art as toner. During development the toner particles are attracted from the carrier particles by the charge pattern of the image areas on the photoconductive insulating area to form a powder image on the photoconductive area. This image may be subsequently transferred to a support surface such as copy paper to which it may be permanently affixed by heating or by the application of pressure. Following transfer of the toner image to the support surface the photoconductive insulating surface may be discharged and cleaned of residual toner to prepare for the next imaging cycle.
In the commercially available electrostatographic reproducing apparatus, attempts are constantly being made to improve the output and in particular, the copy quality of the product produced from such an apparatus. One of the difficulties frequently encountered is the occurrence of streaking in the final copy. By streaking it is intended to define that abnormally high or abnormally low level of toner deposition on the photoconductive surface during the imaging cycle and the subsequent transfer of the toner to the copy sheet. This may occur, for example, as the photoconductive surface, typically in the form of a rotating cylindrical drum, rotates from image cycle to image cycle building up non-uniform charge and therefore non-uniform toner which results in such streaks. For example, in a typical commercial embodiment a photoconductive layer made of a selenium alloy is positively charged and developed with negatively charged toner. Following transfer of toner image in configuration to the copy sheet, it may be discharged by a corona from an AC corotron prior to cleaning for the next imaging cycle. This pre-clean corotron is typically used to remove residual charge on the drum to a zero level to prepare it for the next imaging cycle. Without doing this one would obtain a streaking problem in the copier due to the cyclic history of non-uniform build up. This is particularly magnified if the drum has been used to make 100 copies of an original with the same areas being repeatedly charged and developed, and other areas being greatly fatigued due to exposure in a non-uniform fashion.
An AC corotron generates corona of both a positive and negative phase which tends to be non-uniform along the length of the wire as to current output in the negative phase. Thus, on being discharged, the drum sees the sum of the two phases which is a non-uniform current output or distribution. This locally causes non-uniform discharging on the drum and in subsequent image cycles portions of the drum receive a non-uniform build up of negative charge. Furthermore, once the negative charge has been injected into the selenium alloy drum, they have a tendency to become locallized or bound in the photoconductive layer and tend to dissipate very slowly, thus resulting in a cyclic build up of negative charge in the photoconductive layer. While not wishing to be bound to any theory, it is believed that the negative charge is trapped in the photoconductive layer. Thus on a subsequent cycle, where the photoconductive layer is positvely charged, the negative charged portions of the photoconductor are cyclically built up requiring increasing amounts of positive charge to neutralize them in subsequent imaging cycles. This is in contrast to a positive charge on the selenium alloy which as a charge carrier has such mobility that it goes directly to the conductive substrate on the photoreceptor. By contrast, it is believed that the negative carrier travels 30 to 40 times as slow as the positive charge. Furthermore, and to compound the difficulties, it is believed that any negative charge present on the photoreceptor in subsequent imaging cycles appears to be capable of holding more than its equivalent in positive charge which provides an additional internal positive charge build up in the photoconductor. Finally, the difficulty is compounded if the photoconductor drum is used to repetitively make a large number of copies of the same original in that certain areas are greatly fatigued which causes the drum to charge subsequently in subsequent imaging cycles in a non-uniform fashion which results in more toner being deposited in higher charged areas and eventually streaking in the final copy output.
Many of the above difficulties are traced to the non-uniformity of corona generation along the length of the corotron wire which is positioned in the reproducing apparatus typically so that the photoconductive surface passes parallel to and adjacent to it. Reference to FIG. 1 illustrates the positive and negative phase as well as the difference in charge that may be obtained from a standard AC corona generating device. In the bottom portion of FIG. 1, the positive charge along the length, for example, of an AC corotron is shown as being relatively even and constant. In contrast, the negative phase represented in the top of the graph is shown as being relatively uneven having locallized peaks and valleys. The dashed line in the top of the Figure is a transposition of the positive phase to the negative phase with the hatched area representing the residual negative charge to the photoconductor in the imaging cycle. This difference between the positive and negative charge uniformity in an AC corona generating device is believed to be dependent upon the defects and deficiencies in the wire. It is known that the corona generated along the corona wire varies drastically depending upon the thickness of the wire. A thin wire has a much lower threshold potential and therefore produces a higher corona than a thick wire. It takes longer and more charge for a thicker wire to create the same corona.
Thus the streaking problem is broken into two essential aspects. One the build up even for uniform document input based on non-uniform charging and secondly the effect of repetitive copying of the same original so that the same portions of the photoconductor drum are imaged and discharged on repeated cycles providing a build up of charge in the same areas thereby requiring more toner to develop it which is subsequently transferred to the copy sheet. The gradual build up of trapped charge may even reach a level where it completely changes the cycling imaging characteristics of the drum.
One way of providing a more uniformly controlled charge is with the use of a screen controlled device called a scorotron which consists of one or more fine wires supported on insulated blocks spaced between the photoconductive surface and a grounded conductive surface parallel to it. A screen or grid is interposed between the corona wires and the photoconductive plate and the grid is maintained at a potential roughly equal to the potential desired on the plate. Typically in the scorotrons geometry, the individual wires are from 1/2 to 11/2 inches apart and are spaced from the grid by about 3/4 of an inch. In theory ions from the corona wires will pass between the grid wires and continue on to the plate as long as the potential difference is large between the grid and the plate. When the plate has reached sufficient charge that it is potentially matched to that of the grid charging will cease. While these devices provide good control and excellent reproducibility of potential, they are complex in construction, costly to manufacture, difficult to keep clean and repair, and require power sources for both corona wires and the screen and, are typically bulky occuping considerable space in the machine.